Adaptive filters, whether employed as echo cancelers or otherwise, may require encoded signals internally which are different from those used in a system in which the filter or echo canceler is to be employed. For example, the adaptive filter or echo canceler may use linear or floating point arithmetic to process signals while the system in which it is employed requires .mu.-law PCM. That is to say, the adaptive filter or echo canceler may use signals encoded in a first digital code while the system uses signals encoded in a second digital code.
A typical prior art arrangement employing an adaptive transversal filter as an echo canceler is shown in FIG. 1. In this example, echo canceler 101 including an adaptive transversal filter uses linear arithmetic for processing incoming signals for estimating an impulse response to generate an estimate of an echo signal, while the system uses .mu.-law PCM signals. Accordingly, a signal incoming to equalizer 102 is a linearly encoded digital signal having, for example, 20 bits. An output from equalizer 102 is supplied via gain unit 103 to the X input of echo canceler 101 and to linear-to-.mu.-law converter 104. The .mu.-law PCM signal is 8 bits. Consequently, a nonlinearity is introduced into the return signal path to echo canceler 101. A decoder in CODEC 105 converts the .mu.-law PCM signal into analog form. The analog signal is supplied via hybrid 106 to bidirectional transmission path 107. An analog signal from bidirectional path 107 is supplied via hybrid 106 to a coder in CODEC 105 where it is converted to .mu.-law PCM form. The .mu.-law PCM signal including an echo signal developed in the return signal path from CODEC 105 is supplied to the Y input of echo canceler 101 where it is converted to linear form (a distortion-free conversion) and combined with the echo estimate signal. The combined signal is supplied from the E output of echo canceler 101 for transmission as desired. The so-called return signal path to echo canceler 101, in such an arrangement, includes converter 104, CODEC 105, hybrid 106 and bidirectional path 107.
We have determined that because of the nonlinearity introduced in the return signal path by the truncation of the linear encoded signal by the linear-to-.mu.-law conversion, a "poor" impulse response estimate of the return signal path is obtained. Consequently, a "poor" estimate is obtained of the echo signal to be canceled. This is undesirable. Additionally, when such arrangements are used in voice frequency repeaters, singing, i.e., oscillating, in the repeater may occur and the poor impulse response estimate would slow any recovery from the oscillating condition.